It has long been realized that the force commutated cycloconverter is best suited for suppplying variable frequency AC power to control the speed of AC machines. That it can be advantageously used for other applications in view of its unique properties, has also been established.
A cycloconverter is comprised of a multiplicity of electronic "switches" (e.g., transistors, thyristors) used to fabricate an AC output voltage waveform from a multiphase input source. This is accomplished by sequentially switching segments of the input voltage waves to the output, so that the desired output waveform is produced. The operating principle of a cycloconverter is well-known in the art and is described in a book entitled "Thyristor Phase-Controlled Converters And Cycloconverters" by B. R. Pelly, published in 1971 by John Wiley and Sons. For the force commutated cycloconverter the output waveform is fabricated in such a manner that the "switches" have the ability to interrupt the flow of current at any time independently of the instantaneous input source voltages and of the load current.
In practice, the switches may be realized either by devices having intrinsic turn-off ability (i.e., transistors or gate controlled switches), or SCR devices often called thyristors with the additional force commutating circuitry which is necessary to terminate the conduction of this latter type of switches. Unfortunately, adequately rated "turn-off" devices which are workable in high power systems are not presently available, therefore the second type of switches, e.g. thyristors, must generally be used.
In the last decade considerable effort has been expended to devise techniques for force commutating thyristors in cycloconverters. However, most of the circuits developed to date have serious shortcomings, and are suitable only for rather limited applications. In view of the great potential advantage of the forced commutated cycloconverter, it is desirable to improve on the present commutating circuits, especially for the commutation of the "Unrestricted Frequency Changer" (U.F.C.) with three-phase output, since this system has been shown to be most practical and economical in industrial applications. (For a broad definition of the Unrestricted Frequency Changer see in "Electronics Engineers' Handbook," First Edition, 1975, McGraw-Hill Co., Section 15-42, page 15-52 under "Power Frequency Changers" by L. Gyugyi. A typical unrestricted frequency changer has been described in U.S. Pat. No. 3,170,107 of R. D. Jessee.).
There are basically three approaches to force commutation of the main thyristors of a cycloconverter:
1. Individual switch commutation (i.e., commutating each conducting switch separately), PA1 2. Input line commutation (i.e., commutating all switches at the input lines), PA1 3. Load commutation (i.e., commutating all switches at the load).
These approaches have relative advantages and disadvantages.